The present invention relates generally to electric motors and, more particularly, to a system and method for determining rotor speed of an electric motor.
Electric motors consume a large percentage of generated electricity capacity. Many applications for this “workhorse” of industry are fan and pump industrial applications. For example, in a typical integrated paper mill, low voltage and medium voltage motors may comprise nearly 70% of all driven electrical loads. Due to the prevalence of these motors in industry, it is paramount that the electric motors be operated reliably and efficiently. Motor design parameters and performance parameters are often required by motor management systems to optimize the control and operations of electric motors. Similarly, motor status monitoring enables the electric motors to operate reliably. Many motor status monitoring techniques also look for certain motor design parameters and performance parameters.
One such motor performance parameter that is helpful in optimizing the control and operations of electric motors is rotor or motor speed. Existing motor speed estimation methods for electric motors such as induction motors are typically based on motor equivalent models or complex digital signal processing techniques, such as Fast Fourier Transform (FFT) or other frequency-domain signal processing operations. For low-end motor control or monitoring products, these techniques often cannot be implemented because of hardware and software restrictions. For line-connected motors, the speed may be estimated using a technique that implements a linear speed-load curve derived from rated motor speed (RPM) and synchronous speed (RPM). However, for inverter-fed motors such as variable frequency drive (VFD) motors, such motor speed estimation often cannot be used since the rated RPM in the nameplate is only valid for rated motor operations (e.g., at a rated voltage and a rated frequency).
It would therefore be desirable to design a system and method for determining motor speed of an electric motor that is not dependent on set load, voltage, and frequency conditions, so as to enable the improved motor management and motor status monitoring for inverter-fed motors.